Various types of non-contact distance measuring devices are presently available for measuring linear and angular displacement and detecting absolute position of an object in linear or angular systems. A classic example is a sliding resistor potentiometer which, as known to those skilled in the art, is generally unreliable. Optical position devices are also available which utilize optical sensors to provide an optical scale such as a slit scale. While generally more reliable than the aforementioned resistor potentiometer instruments, optical position devices nonetheless require complicated construction and are thus expensive to manufacture and are somewhat difficult to use. They are also very sensitive to dirt and other contaminants and have been found in practice very difficult to keep clean.
There are also available magnetic scales wherein a scale written into a magnetic medium is read out by a magnetic sensor. Again, however, these devices require fairly complicated structure and are thus expensive to manufacture and difficult to use. Moreover, these magnetic scales are incapable of measuring absolute position. As those skilled in the art will recognize, such devices are only operative to determine the distance between two arbitrary points.
One attempt to overcome the aforementioned difficulties associated with the prior art devices is disclosed in U.S. Pat. No. 4,810,965 issued to Fujiwara et al. and entitled "Position Detecting Apparatus Using A Magnetic Sensor And A Closed Magnetic Circuit With Non-Uniform Magnetic Flux Distribution". The '965 patent is assigned to Fujitsu, Ltd., Kawasaki, Japan. Fujiwara discloses a magnetic closed circuit having many magnetic and mechanical configurations directed to sensing linear or angular displacement of a desired object. In operation, the device disclosed by Fujiwara sets up a magnetic circuit which gives a magnetic field whose intensity depends upon position. Properties of the circuit, however, do not vary. As shown, the magnetic field sensor is attached to the part whose position is sought to be determined and is also in the spatially varying magnetic field. Thus, changing the sensor's position changes the field to which it is exposed.
Consequently, a need has developed for an improved contactless position detecting apparatus and method. Such an apparatus and method should permit the angular or linear position of a desired object to be determined with high reliability and should further be relatively inexpensive to manufacture and use compared to prior art devices.